Nowadays, more than one Global Navigation Satellite System (GNSS) is available. A receiver supporting multi-specification LBS (location based service), wireless multimedia communication and broadcasting signals is becoming an expectation. Take multi-specification LBS as an example, such a receiver able to support multi-mode receiving for GNSS signals can enhance locating precision and access to more services. Among the GNSS systems, different signal frequency bands support different services. As more and more bands need to be supported, band overlapping occurs.
GPS is the U.S. navigation satellite system, which is a network of satellites continuously transmits high-frequency radio signals. The signals carry time and distance information that is receivable by a GPS receiver, so that a user can pinpoint the position thereof on the earth. Galileo, the emerging European satellite navigation system, offers higher signal power and more robust modulation that will enable users to receive weak signals even in difficult environments. When combined, Galileo and GPS will offer twice the number of satellite sources as currently available. This provides redundancy as well as greater availability for the user. The combination of GPS and Galileo basically has four bands, excluding SAR (Safe and Rescue) service. GPS and Galileo systems share some signal bands. That is, GPS and Galileo share some central frequencies and send signals on the same ones of carriers. For example, GPS L1 and Galileo E2-L1-E1 share the same band. To reduce inter-system and intra-system interference, specific modulation schemes are required. Binary offset carrier modulation (hereinafter simply referred to as “BOC”) is a widely used method.
The BOC modulation is done by multiplying a pseudo-random noise (PRN) spreading coded signal (simply referred to as PRN coded signal hereinafter) with a square wave subcarrier (SC). The SC has a frequency which is multiple of the code rate of the PRN spreading code. FIG. 1 is a waveform diagram showing the BOC modulation. The BOC-sine (simply referred to as BOC) signal is generated by mixing a SC-sine and a PRN coded signal, while the BOC-cos (also referred to as QBOC, where Q indicates “quadrature-phase”.) is generated by mixing an SC-cos and the PRN coded signal.
The BOC signal has a symmetric split spectrum with two main lobes shifted from the center frequency by the frequency of the subcarrier. The characteristics of the BOC signal are dependent on the spreading code chip rate, the subcarrier frequency, and the subcarrier phasing within one PRN code chip. The common notation for a BOC-modulated signals in the GNSS field is represented as BOC(fc, fs), where fc is the code chip rate, and L is the frequency of the subcarrier. Both fc and fs are usually represented as a multiple of the reference frequency 1.023 MHz. Therefore, the BOC signal can also be represented as BOC(n,m), where n is the multiple of 1.023 MHz for the PRN code chip rate fc, and m is the multiple of 1.023 MHz for the subcarrier fs.
For satellite signal navigation, the BOC signal is preferably applied in tracking under white noises. Such scheme provides better inherent multipath mitigation compared to the spreading code alone. However, BOC scheme makes acquisition and tracking more difficult due to a multiple peak autocorrelation phenomenon. The presence of the subcarrier in the BOC signal introduces secondary peaks in a range of −1/+1 chip in BOC autocorrelation. FIG. 2 is a diagram showing autocorrelation of BOC(1,1). That is, BOC(1,1) correlates with BOC(1,1). As shown, there are two troughs at both sides of the main peak in the middle. To calculate correlation power, square of correlation is usually used. Accordingly, the two troughs will cause two secondary peaks in view of correlation power. Such secondary peaks may cause a problem of mis-lock. That is, a receiver may lock the secondary peak rather than the main peak, and therefore resulting in erroneous tracking. A significant deviation of approximately 150 m would occur in the range measurement. Such an error is unacceptable in navigation.
In addition, the width of the main lobe (main peak) of the BOC correlation result influences the performance of the receiver in acquisition and tracking. If the main lobe is narrow, it is good for tracking and position because a more accurate code phase can be tracked. However, a narrow main lobe makes it difficult to acquire the signal because the narrow correlation function leads to a finer code phase searching space, which needs longer acquisition time.